It is prior known in combustion apparatus employing admixed a gaseous fuel and primary combustion air to employ one or more sensing elements for either indicating the presence of excess oxygen in the burner flame, such as will ensure complete combustion of the gaseous fuel with maximum efficiency, or, for indicating the presence of an oxygen deficiency in the products of combustion, such as would result in incomplete and inefficient combustion of the gaseous fuel.
Such sensing elements commonly employ a substrate of a material, such as zirconia, having electrodes of a rare earth metal, such as platinum, on opposite faces thereof, such an element having the capability of developing an electro-motive force across the electrodes in the presence of oxygen by ion exchange at such time as the substrate is heated to a temperature sufficient to reduce the internal electrical resistance of the substrate. Alternatively, the substrate may be formed from titanium, which, upon heating of the substrate to a determined temperature, changes in electrical resistance in direct relationship to the concentration of oxygen present in the combustion products.
The change in electro-motive force in the former type of element, and the change in internal resistance in the latter type of element is employed to control electronic circuitry, which may be employed to effect automatic control of the fuel to air ratio, or, in extreme circumstances, to shut down operation of the combustion apparatus in the event that a potentially dangerous condition exists.
While such an apparatus employing a single element finds application in those circumstances where the gaseous fuel supplied to the combustion apparatus is unchanging in combustion characteristics, problems arise when a gaseous fuel is substituted for an existing fuel supply, in which the substituted fuel has combustion characteristics different from that of the said existing fuel supply.
In order to accomodate change-over in the supply of one gaseous fuel to another, it has been prior proposed to employ two identical sensing elements which are usable either singly or in tandem to control the electronic control circuitry, and which are selectively switchable into the electronic control circuitry in dependence on the combustion characteristics of the fuel supply employed at a particular time.
The provision of dual independent sensing elements, while satisfactory in many respects, is encumbered with the problem of additional expense in the provision of such dual elements, and also is encumbered with the problem of accurately positioning the respective elements relative to the burner in closely adjacent position in order to minimize inaccuracies in detection by the respective elements.